The invention relates to a cathode-ray tube comprising, in an evacuated envelope, a diode electron gun for generating an electron beam. The electron gun comprises a cathode, which is arranged on an axis with its emissive surface extending substantially perpendicularly to the axis, and an anode extending substantially perpendicularly to the axis and having an aperture situated opposite to the cathode. The electron beam is focused on a target by means of at least one focusing lens.
Such a cathode-ray tube is known from U.S. Pat. No. 3,831,058 which discloses a television camera tube having a diode electron gun. Because no cross-over is formed in the electron beam produced by this gun, the beam current inertia is reduced due to the decrease of the interactions between the electrons. A part of the anode including an aperture having a radius of 0.01 mm is spaced from the cathode by 0.5 mm. The electron beam in a television camera tube is not modulated. The beam current in such a tube is a few to a few tens of micro-amperes.
Most of the known cathode-ray tubes for displaying pictures, such as colour and black-and-white display tubes, projection television display tubes, data graphic display (D.G.D.) tubes, and oscilloscope tubes, comprise a triode electron gun having a cathode, a negative grid and an anode. In such a triode electron gun a cross-over is formed between the cathode and the anode and is displayed on the display screen of the cathode-ray tube by means of one or more focusing lenses. The electron beam is modulated by a voltage variation at the cathode (cathode control) or at the negative grid (grid control). In such a triode electron gun the modulation and the electron beam formation are coupled. Upon forming the cross-over, aberrations are formed in the electron beam which result in an enlargement of the spot on the display screen. Such aberrations occur much less in a diode electron gun. However, for a number of reasons it is not possible to use the known diode electron gun in a picture display tube. As is known, the electron beam current in a picture display tube is much larger than in a television camera tube and, dependent on the type of tube, is 0.01- 5 mA. With these electron beam currents the dissipation in the anode would become much too large. Moreover, without cross-over formation it is substantially impossible to adapt the beam aperture angle optimally to the main focusing lens.